pkg/transport: reload TLS certificates for every use

This changes the baseConfig used when creating tls Configs to utilize
the GetCertificate and GetClientCertificate functions to always reload
the certificates from disk whenever they are needed.

Always reloading the certificates allows changing the certificates via
an external process without interrupting etcd.

Fixes etcd-io#7576

pkg/transport/listener.go

@@ -172,6 +172,14 @@ func (info TLSInfo) baseConfig() (*tls.Config, error) {
 		MinVersion:   tls.VersionTLS12,
 		ServerName:   info.ServerName,
 	}
+	// TODO: this loads certificate from disk for every client request
+	// support server-side refresh (e.g. fsnotify, SIGHUP), caching
+	cfg.GetCertificate = func(clientHello *tls.ClientHelloInfo) (*tls.Certificate, error) {
+		return tlsutil.NewCert(info.CertFile, info.KeyFile, info.parseFunc)
+	}
+	cfg.GetClientCertificate = func(unused *tls.CertificateRequestInfo) (*tls.Certificate, error) {
+		return tlsutil.NewCert(info.CertFile, info.KeyFile, info.parseFunc)
+	}
 	return cfg, nil
 }